Silver is chemically the most reactive element among the noble metals and tarnishes readily on exposure to sulfur bearing atmospheres. Discoloration, generally termed tarnishing, is caused by a silver oxidation process in which sulfide is formed. Food, such as onions, mustard and eggs that contain organic sulfur compounds, are also known to tarnish silver. See Singh et al., "Silver Tarnishing and its Prevention--A Review" Anti-corrosion Methods and Material, v. 30 (July 1983) pp. 4-8.
Bleaching compositions and bleach systems are well known and provide desired cleaning properties in many commercial detergents. Chlorine and N,N,N',N'-tetraacetyl ethylene diamine (TAED)/perborate, for example, are well known for their bleaching properties. Bleaching systems that include cationic nitriles in the presence of peroxide are also known (see, for example, U.S. Pat. Nos. 5,236,616 and 5,281,361, EP 0 303 520 B1 and WO 99/63038, the contents of which are incorporated herein by reference).
Silver tarnishing is also known to occur when an oxygen bleaching agent used in detergent compositions oxidizes the silver to silver oxide. This oxidation process causes surface blackening of the silverware when machine dishwashed.
Conventional detergents, particularly automatic machine dishwashing detergents, are generally formulated with chlorine bleaching agents in a high alkaline pH range. During washing, certain chlorine bleaches (e.g., chloroisocyanurate) react to form isocyanuric acid and thus not greatly effect silver discolorization.
Detergent compositions are, however, increasingly being based on peroxygen bleaching agents and are being formulated to be milder to produce environmentally friendly products. The problem of tarnishing of silver and silver plated articles has thus become more severe.
Therefore, there is a need for detergent compositions that have bleaching compositions yet have minimal detrimental effects on silverware. There is also a need for more cost-efficient detergent formulations, wherein the desired benefits are achieved through decreased quantities of raw material, through the use of less expensive ingredients and/or more efficient compatibility of materials.